The present invention relates to an optical recording/reproducing apparatus for a multi-layer recording medium having a plurality of stacked information recording layers, which irradiates each of the recording layers with a condensed light beam to record and/or reproduce information to and/or from the multi-layer recording medium.
In recent years, to increase the recording density of recording media, measures such as increasing the line recording density, decreasing the track pitch, etc. have been taken. To increase the recording density, it is necessary to reduce the diameter of a light beam condensed on an information recording layer of a recording medium. As approaches to reduction of the beam diameter, techniques of increasing the numerical aperture (NA) of a light beam outgoing from an objective lens provided in an optical recording/reproducing apparatus, making the wavelength of such a light beam shorter, etc. have been developed.
A multi-layer recording medium, which has a plurality of stacked information recording layers so as to have an increased information recording density in the direction of thickness of the recording medium, has already been commercialized as a double layer disc of DVD (digital versatile disc) format. The optical recording/reproducing apparatus for such multi-layer recording media is required to condense a light beam to a sufficiently small spot on each of the recording layers.
The different recording layers of the recording medium, however, have different spherical aberrations (SA) which occur while the light beam passes through a cover glass of the recording medium. For example, a difference ΔSA between the spherical aberrations of adjacent recording layers is proportional to Δt and a biquadrate of NA (numerical aperture), as indicated by the following expression (1).ΔSA∝Δt×NA4  (1)where Δt is a distance between the adjacent recording layers.
When the numerical aperture NA of the objective lens is around 0.6, a value of ΔSA (difference between the spherical aberrations of adjacent recording layers) is comparatively small as in conventional DVD discs. Therefore, it was possible to condense a light beam to a sufficiently small spot on each recording layer.
If the numerical aperture NA is increased up to, for example, 0.85, the spherical aberration difference ΔSA becomes about 4 times larger than when NA=0.6. Accordingly, the spherical aberrations of each of the recording layers become larger. For this reason, it is difficult to sufficiently converge a light beam on each of the recording layers.